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The Linux / Unix Command: expect

Official Software Documentation (Man Page) Reformatted



expect - programmed dialogue with interactive programs, Version 5


expect [ -dDinN ] [ -c cmds ] [ [ - [ f | b ] ] cmdfile ] [ args ]


Expect is a program that "talks" to other interactive programs according to a script. Following the script, Expect knows what can be expected from a program and what the correct response should be. An interpreted language provides branching and high-level control structures to direct the dialogue. In addition, the user can take control and interact directly when desired, afterward returning control to the script.

Expectk is a mixture of Expect and Tk . It behaves just like Expect and Tk 's wish . Expect can also be used directly in C or C++ (that is, without Tcl). See libexpect(3).

The name "Expect" comes from the idea of send/expect sequences popularized by uucp, kermit and other modem control programs. However unlike uucp, Expect is generalized so that it can be run as a user-level command with any program and task in mind. Expect can actually talk to several programs at the same time.

For example, here are some things Expect can do:

  • Cause your computer to dial you back, so that you can login without paying for the call.
  • Start a game (e.g., rogue) and if the optimal configuration doesn't appear, restart it (again and again) until it does, then hand over control to you.
  • Run fsck, and in response to its questions, answer "yes", "no" or give control back to you, based on predetermined criteria.
  • Connect to another network or BBS (e.g., MCI Mail, CompuServe) and automatically retrieve your mail so that it appears as if it was originally sent to your local system.
  • Carry environment variables, current directory, or any kind of information across rlogin, telnet, tip, su, chgrp, etc.

There are a variety of reasons why the shell cannot perform these tasks. (Try, you'll see.) All are possible with Expect .

In general, Expect is useful for running any program which requires interaction between the program and the user. All that is necessary is that the interaction can be characterized programmatically. Expect can also give the user back control (without halting the program being controlled) if desired. Similarly, the user can return control to the script at any time.


Expect reads cmdfile for a list of commands to execute. Expect may also be invoked implicitly on systems which support the #! notation by marking the script executable, and making the first line in your script:

#!/usr/local/bin/expect -f

Of course, the path must accurately describe where Expect lives. /usr/local/bin is just an example.

The -c flag prefaces a command to be executed before any in the script. The command should be quoted to prevent being broken up by the shell. This option may be used multiple times. Multiple commands may be executed with a single -c by separating them with semicolons. Commands are executed in the order they appear. (When using Expectk, this option is specified as -command .)

The -d flag enables some diagnostic output, which primarily reports internal activity of commands such as expect and interact . This flag has the same effect as "exp_internal 1" at the beginning of an Expect script, plus the version of Expect is printed. (The strace command is useful for tracing statements, and the trace command is useful for tracing variable assignments.) (When using Expectk, this option is specified as -diag .)

The -D flag enables an interactive debugger. An integer value should follow. The debugger will take control before the next Tcl procedure if the value is non-zero or if a ^C is pressed (or a breakpoint is hit, or other appropriate debugger command appears in the script). See the README file or SEE ALSO (below) for more information on the debugger. (When using Expectk, this option is specified as -Debug .)

The -f flag prefaces a file from which to read commands from. The flag itself is optional as it is only useful when using the #! notation (see above), so that other arguments may be supplied on the command line. (When using Expectk, this option is specified as -file .)

By default, the command file is read into memory and executed in its entirety. It is occasionally desirable to read files one line at a time. For example, stdin is read this way. In order to force arbitrary files to be handled this way, use the -b flag. (When using Expectk, this option is specified as -buffer .) Note that stdio-buffering may still take place however this shouldn't cause problems when reading from a fifo or stdin.

If the string "-" is supplied as a filename, standard input is read instead. (Use "./-" to read from a file actually named "-".)

The -i flag causes Expect to interactively prompt for commands instead of reading them from a file. Prompting is terminated via the exit command or upon EOF. See interpreter (below) for more information. -i is assumed if neither a command file nor -c is used. (When using Expectk, this option is specified as -interactive .)

-- may be used to delimit the end of the options. This is useful if you want to pass an option-like argument to your script without it being interpreted by Expect . This can usefully be placed in the #! line to prevent any flag-like interpretation by Expect. For example, the following will leave the original arguments (including the script name) in the variable argv .

#!/usr/local/bin/expect --

Note that the usual getopt(3) and execve(2) conventions must be observed when adding arguments to the #! line.

The file $exp_library/expect.rc is sourced automatically if present, unless the -N flag is used. (When using Expectk, this option is specified as -NORC .) Immediately after this, the file ~/.expect.rc is sourced automatically, unless the -n flag is used. If the environment variable DOTDIR is defined, it is treated as a directory and .expect.rc is read from there. (When using Expectk, this option is specified as -norc .) This sourcing occurs only after executing any -c flags.

-v causes Expect to print its version number and exit. (The corresponding flag in Expectk, which uses long flag names, is -version.)

Optional args are constructed into a list and stored in the variable named argv . argc is initialized to the length of argv.

argv0 is defined to be the name of the script (or binary if no script is used). For example, the following prints out the name of the script and the first three arguments:

    send_user "$argv0 [lrange $argv 0 2]\n"


Expect uses Tcl (Tool Command Language). Tcl provides control flow (e.g., if, for, break), expression evaluation and several other features such as recursion, procedure definition, etc. Commands used here but not defined (e.g., set , if , exec ) are Tcl commands (see tcl(3)). Expect supports additional commands, described below. Unless otherwise specified, commands return the empty string.

Commands are listed alphabetically so that they can be quickly located. However, new users may find it easier to start by reading the descriptions of spawn , send , expect , and interact , in that order.

Note that the best introduction to the language (both Expect and Tcl) is provided in the book "Exploring Expect" (see SEE ALSO below). Examples are included in this man page but they are very limited since this man page is meant primarily as reference material.

Note that in the text of this man page, "Expect" with an uppercase "E" refers to the Expect program while "expect" with a lower-case "e" refers to the expect command within the Expect program.) .I

close [-slave] [-onexec 0|1] [-i spawn_id]
closes the connection to the current process. Most interactive programs will detect EOF on their stdin and exit; thus close usually suffices to kill the process as well. The -i flag declares the process to close corresponding to the named spawn_id.

Both expect and interact will detect when the current process exits and implicitly do a close . But if you kill the process by, say, "exec kill $pid", you will need to explicitly call close.

The -onexec flag determines whether the spawn id will be closed in any new spawned processes or if the process is overlayed. To leave a spawn id open, use the value 0. A non-zero integer value will force the spawn closed (the default) in any new processes.

The -slave flag closes the slave associated with the spawn id. (See "spawn -pty".) When the connection is closed, the slave is automatically closed as well if still open.

No matter whether the connection is closed implicitly or explicitly, you should call wait to clear up the corresponding kernel process slot. close does not call wait since there is no guarantee that closing a process connection will cause it to exit. See wait below for more info.

debug [[-now] 0|1]
controls a Tcl debugger allowing you to step through statements, set breakpoints, etc.

With no arguments, a 1 is returned if the debugger is not running, otherwise a 0 is returned.

With a 1 argument, the debugger is started. With a 0 argument, the debugger is stopped. If a 1 argument is preceded by the -now flag, the debugger is started immediately (i.e., in the middle of the debug command itself). Otherwise, the debugger is started with the next Tcl statement.

The debug command does not change any traps. Compare this to starting Expect with the -D flag (see above).

See the README file or SEE ALSO (below) for more information on the debugger.

disconnects a forked process from the terminal. It continues running in the background. The process is given its own process group (if possible). Standard I/O is redirected to /dev/null.

The following fragment uses disconnect to continue running the script in the background.

    if {[fork]!=0} exit
    . . .
The following script reads a password, and then runs a program every hour that demands a password each time it is run. The script supplies the password so that you only have to type it once. (See the stty command which demonstrates how to turn off password echoing.)
    send_user "password?\ "
    expect_user -re "(.*)\n"
    for {} 1 {} {
        if {[fork]!=0} {sleep 3600;continue}
        spawn priv_prog
        expect Password:
        send "$expect_out(1,string)\r"
        . . .
An advantage to using disconnect over the shell asynchronous process feature (&) is that Expect can save the terminal parameters prior to disconnection, and then later apply them to new ptys. With &, Expect does not have a chance to read the terminal's parameters since the terminal is already disconnected by the time Expect receives control.

exit [-opts] [status]
causes Expect to exit or otherwise prepare to do so.

The -onexit flag causes the next argument to be used as an exit handler. Without an argument, the current exit handler is returned.

The -noexit flag causes Expect to prepare to exit but stop short of actually returning control to the operating system. The user-defined exit handler is run as well as Expect's own internal handlers. No further Expect commands should be executed. This is useful if you are running Expect with other Tcl extensions. The current interpreter (and main window if in the Tk environment) remain so that other Tcl extensions can clean up. If Expect's exit is called again (however this might occur), the handlers are not rerun.

Upon exiting, all connections to spawned processes are closed. Closure will be detected as an EOF by spawned processes. exit takes no other actions beyond what the normal _exit(2) procedure does. Thus, spawned processes that do not check for EOF may continue to run. (A variety of conditions are important to determining, for example, what signals a spawned process will be sent, but these are system-dependent, typically documented under exit(3).) Spawned processes that continue to run will be inherited by init.

status (or 0 if not specified) is returned as the exit status of Expect . exit is implicitly executed if the end of the script is reached.

exp_continue [-continue_timer]
The command exp_continue allows expect itself to continue executing rather than returning as it normally would. By default exp_continue resets the timeout timer. The -continue_timer flag prevents timer from being restarted. (See expect for more information.)

exp_internal [-f file] value
causes further commands to send diagnostic information internal to Expect to stderr if value is non-zero. This output is disabled if value is 0. The diagnostic information includes every character received, and every attempt made to match the current output against the patterns.

If the optional file is supplied, all normal and debugging output is written to that file (regardless of the value of value ). Any previous diagnostic output file is closed.

The -info flag causes exp_internal to return a description of the most recent non-info arguments given.

exp_open [args] [-i spawn_id]
returns a Tcl file identifier that corresponds to the original spawn id. The file identifier can then be used as if it were opened by Tcl's open command. (The spawn id should no longer be used. A wait should not be executed.

The -leaveopen flag leaves the spawn id open for access through Expect commands. A wait must be executed on the spawn id.

exp_pid [-i spawn_id]
returns the process id corresponding to the currently spawned process. If the -i flag is used, the pid returned corresponds to that of the given spawn id.

is an alias for send .

is an alias for send_error .

is an alias for send_log .

is an alias for send_tty .

is an alias for send_user .

exp_version [[-exit] version]
is useful for assuring that the script is compatible with the current version of Expect.

With no arguments, the current version of Expect is returned. This version may then be encoded in your script. If you actually know that you are not using features of recent versions, you can specify an earlier version.

Versions consist of three numbers separated by dots. First is the major number. Scripts written for versions of Expect with a different major number will almost certainly not work. exp_version returns an error if the major numbers do not match.

Second is the minor number. Scripts written for a version with a greater minor number than the current version may depend upon some new feature and might not run. exp_version returns an error if the major numbers match, but the script minor number is greater than that of the running Expect .

Third is a number that plays no part in the version comparison. However, it is incremented when the Expect software distribution is changed in any way, such as by additional documentation or optimization. It is reset to 0 upon each new minor version.

With the -exit flag, Expect prints an error and exits if the version is out of date.

expect [[-opts] pat1 body1] ... [-opts] patn [bodyn]
waits until one of the patterns matches the output of a spawned process, a specified time period has passed, or an end-of-file is seen. If the final body is empty, it may be omitted.

Patterns from the most recent expect_before command are implicitly used before any other patterns. Patterns from the most recent expect_after command are implicitly used after any other patterns.

If the arguments to the entire expect statement require more than one line, all the arguments may be "braced" into one so as to avoid terminating each line with a backslash. In this one case, the usual Tcl substitutions will occur despite the braces.

If a pattern is the keyword eof , the corresponding body is executed upon end-of-file. If a pattern is the keyword timeout , the corresponding body is executed upon timeout. If no timeout keyword is used, an implicit null action is executed upon timeout. The default timeout period is 10 seconds but may be set, for example to 30, by the command "set timeout 30". An infinite timeout may be designated by the value -1. If a pattern is the keyword default , the corresponding body is executed upon either timeout or end-of-file.

If a pattern matches, then the corresponding body is executed. expect returns the result of the body (or the empty string if no pattern matched). In the event that multiple patterns match, the one appearing first is used to select a body.

Each time new output arrives, it is compared to each pattern in the order they are listed. Thus, you may test for absence of a match by making the last pattern something guaranteed to appear, such as a prompt. In situations where there is no prompt, you must use timeout (just like you would if you were interacting manually).

Patterns are specified in three ways. By default, patterns are specified as with Tcl's string match command. (Such patterns are also similar to C-shell regular expressions usually referred to as "glob" patterns). The -gl flag may may be used to protect patterns that might otherwise match expect flags from doing so. Any pattern beginning with a "-" should be protected this way. (All strings starting with "-" are reserved for future options.)

For example, the following fragment looks for a successful login. (Note that abort is presumed to be a procedure defined elsewhere in the script.)

    expect {
        busy               {puts busy\n ; exp_continue}
        failed             abort
        "invalid password" abort
        timeout            abort
Quotes are necessary on the fourth pattern since it contains a space, which would otherwise separate the pattern from the action. Patterns with the same action (such as the 3rd and 4th) require listing the actions again. This can be avoid by using regexp-style patterns (see below). More information on forming glob-style patterns can be found in the Tcl manual.

Regexp-style patterns follow the syntax defined by Tcl's regexp (short for "regular expression") command. regexp patterns are introduced with the flag -re . The previous example can be rewritten using a regexp as:

    expect {
        busy       {puts busy\n ; exp_continue}
        -re "failed|invalid password" abort
        timeout    abort
Both types of patterns are "unanchored". This means that patterns do not have to match the entire string, but can begin and end the match anywhere in the string (as long as everything else matches). Use ^ to match the beginning of a string, and $ to match the end. Note that if you do not wait for the end of a string, your responses can easily end up in the middle of the string as they are echoed from the spawned process. While still producing correct results, the output can look unnatural. Thus, use of $ is encouraged if you can exactly describe the characters at the end of a string.

Note that in many editors, the ^ and $ match the beginning and end of lines respectively. However, because expect is not line oriented, these characters match the beginning and end of the data (as opposed to lines) currently in the expect matching buffer. (Also, see the note below on "system indigestion.")

The -ex flag causes the pattern to be matched as an "exact" string. No interpretation of *, ^, etc is made (although the usual Tcl conventions must still be observed). Exact patterns are always unanchored.

The -nocase flag causes uppercase characters of the output to compare as if they were lowercase characters. The pattern is not affected.

While reading output, more than 2000 bytes can force earlier bytes to be "forgotten". This may be changed with the function match_max . (Note that excessively large values can slow down the pattern matcher.) If patlist is full_buffer , the corresponding body is executed if match_max bytes have been received and no other patterns have matched. Whether or not the full_buffer keyword is used, the forgotten characters are written to expect_out(buffer).

If patlist is the keyword null , and nulls are allowed (via the remove_nulls command), the corresponding body is executed if a single ASCII 0 is matched. It is not possible to match 0 bytes via glob or regexp patterns.

Upon matching a pattern (or eof or full_buffer), any matching and previously unmatched output is saved in the variable expect_out(buffer) . Up to 9 regexp substring matches are saved in the variables expect_out(1,string) through expect_out(9,string) . If the -indices flag is used before a pattern, the starting and ending indices (in a form suitable for lrange ) of the 10 strings are stored in the variables expect_out(X,start) and expect_out(X,end) where X is a digit, corresponds to the substring position in the buffer. 0 refers to strings which matched the entire pattern and is generated for glob patterns as well as regexp patterns. For example, if a process has produced output of "abcdefgh\n", the result of:

    expect "cd"
is as if the following statements had executed:
    set expect_out(0,string) cd
    set expect_out(buffer) abcd
and "efgh\n" is left in the output buffer. If a process produced the output "abbbcabkkkka\n", the result of:
    expect -indices -re "b(b*).*(k+)"
is as if the following statements had executed:
    set expect_out(0,start) 1
    set expect_out(0,end) 10
    set expect_out(0,string) bbbcabkkkk
    set expect_out(1,start) 2
    set expect_out(1,end) 3
    set expect_out(1,string) bb
    set expect_out(2,start) 10
    set expect_out(2,end) 10
    set expect_out(2,string) k
    set expect_out(buffer) abbbcabkkkk
and "a\n" is left in the output buffer. The pattern "*" (and -re ".*") will flush the output buffer without reading any more output from the process.

Normally, the matched output is discarded from Expect's internal buffers. This may be prevented by prefixing a pattern with the -notransfer flag. This flag is especially useful in experimenting (and can be abbreviated to "-not" for convenience while experimenting).

The spawn id associated with the matching output (or eof or full_buffer) is stored in expect_out(spawn_id) .

The -timeout flag causes the current expect command to use the following value as a timeout instead of using the value of the timeout variable.

By default, patterns are matched against output from the current process, however the -i flag declares the output from the named spawn_id list be matched against any following patterns (up to the next -i ). The spawn_id list should either be a whitespace separated list of spawn_ids or a variable referring to such a list of spawn_ids.

For example, the following example waits for "connected" from the current process, or "busy", "failed" or "invalid password" from the spawn_id named by $proc2.

    expect {
        -i $proc2 busy {puts busy\n ; exp_continue}
        -re "failed|invalid password" abort
        timeout abort
The value of the global variable any_spawn_id may be used to match patterns to any spawn_ids that are named with all other -i flags in the current expect command. The spawn_id from a -i flag with no associated pattern (i.e., followed immediately by another -i ) is made available to any other patterns in the same expect command associated with any_spawn_id.

The -i flag may also name a global variable in which case the variable is read for a list of spawn ids. The variable is reread whenever it changes. This provides a way of changing the I/O source while the command is in execution. Spawn ids provided this way are called "indirect" spawn ids.

Actions such as break and continue cause control structures (i.e., for , proc ) to behave in the usual way. The command exp_continue allows expect itself to continue executing rather than returning as it normally would.

This is useful for avoiding explicit loops or repeated expect statements. The following example is part of a fragment to automate rlogin. The exp_continue avoids having to write a second expect statement (to look for the prompt again) if the rlogin prompts for a password.

    expect {
        Password: {
            stty -echo
            send_user "password (for $user) on $host: "
            expect_user -re "(.*)\n"
            send_user "\n"
            send "$expect_out(1,string)\r"
            stty echo
        } incorrect {
            send_user "invalid password or account\n"
        } timeout {
            send_user "connection to $host timed out\n"
        } eof {
            send_user \
                "connection to host failed: $expect_out(buffer)"
        } -re $prompt
For example, the following fragment might help a user guide an interaction that is already totally automated. In this case, the terminal is put into raw mode. If the user presses "+", a variable is incremented. If "p" is pressed, several returns are sent to the process, perhaps to poke it in some way, and "i" lets the user interact with the process, effectively stealing away control from the script. In each case, the exp_continue allows the current expect to continue pattern matching after executing the current action.
    stty raw -echo
    expect_after {
        -i $user_spawn_id
        "p" {send "\r\r\r"; exp_continue}
        "+" {incr foo; exp_continue}
        "i" {interact; exp_continue}
        "quit" exit

By default, exp_continue resets the timeout timer. The timer is not restarted, if exp_continue is called with the -continue_timer flag.

expect_after [expect_args]
works identically to the expect_before except that if patterns from both expect and expect_after can match, the expect pattern is used. See the expect_before command for more information.

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